He was an opponent of the ideas of Ernst Haeckel.

In 1863, he became professor of zoölogy at Marburg, in 1870 at Göttingen and in 1873 at Vienna.

[1] He was head of the oceanographic research station in Trieste and was specialized on marine zoology and there his interest was focused on crustaceans.

During his research on cell biology he coined the word phagocyte.

He is known for the fact that Sigmund Freud started his studies on the yet unsolved eel life history.